Method of sintering ores



United States Patent G METHOD OF SINTERING ORES Robin Adair Russell, Columbia, Tenn., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Application December 14, 1954, Serial No. 475,273

8 Claims. (Cl. 75-5) This invention relates to a novel method of sintering metal bearing materials, such as ores, concentrates, and the like, to effect agglomeration of the particles. More particularly this invention is directed to an improved method for agglomerating finely divided phosphatic materials by blast roasting on a Dwight and Lloyd sintering machine. More particularly still the improved feature of the instant invention pertains to the preparation of the feed stock for the sintering machine wherein the recycled fines and fuel are mixed therein after the balance of the feed stock has been passed through a pug mill.

The depletion of extensive deposits of the rock phosphate necessitates the working of low-grade deposits which often requires a beneficiation process .to remove excessive clay gangue, etc. Also a large amount of fines are obtained even in working high-grade rock phosphate. Accordingly, it is becoming increasingly important in the phosphorus industry to improve the various means of agglomerating fines to provide a furnaceable material.

The term recycle fines as employed herein is that fraction of the sintered product having a particle size of less than about and greater than about 8 mesh which results from the crushing of the sintered product to obtain the desired furnaceable material of agglomerates from about up to about 3". The minus S-mesh sinter recycle material is called sinter dust and is returned to storage and ultimately used as feed for the pug mill, e. g., along with the matrix and concentrates.

The practice of sintering finely divided, metal-bearing materials to effect agglomeration thereby producing a product of suitable form for charging to a furnace whereby the desired metal can be obtained is Well known to the art. The Dwight and Lloyd sintering machine is employed in many industrial operations and does not require detailed description in view of the general familiarity therewith by those skilled in the art. Briefly the Dwight and Lloyd sintering machine comprises a main frame of suitable construction carrying tracks whereon a plurality of closely fitting pans, buckets or pallets are caused to travel in contiguous relationship one to another at a predetermined rate. The tracks comprise an upper run and a lower run whereby the pallets are continuously cycled by providing the machine with suitable means, e. g., sprocket Wheels, for elevating the pallets from the lower run to the upper run at the feed end of the machine and propelling them along the latter. The pallets are provided with grates thereby forming a continuous traveling grate structure on the upper run of the machine. As the pallets travel along the upper run of the track they first pass beneath a feed hopper by means of which the feed material to be sintered is delivered to the pallets and the surface leveled off or formed in a suitable contour by a gate member in the continued travel of the pallets. Thereafter the loaded pallets pass beneath a suitable device for igniting the feed material, e. g., a hood provided with a plurality of flames which are gently played on the surface of the feed material so as to cause substantially no change in surface 2,794,729 Patented June 4, 1957 contour, while the pallet is over a wind box providing a down-draft through the material. The ignited material continues to travel over a series of wind boxes which are connected to suitable suction means for creating a downdraft of air through the material whereby a relatively thin combustionrzone is maintained and caused to be propagated downwardly through the bed at progressively greater depths along the conveyor until the combustion zone has progressed through to the bottom of the bed assuring complete sintering thereof by the time the pallet reaches the discharge end of the machine. When the pallets reach the discharge end of the sinter machine they pass downwardly and become inverted on the lower return-run tracks whereby the sintered material is discharged from the pallets. The sintered material passes through a crusher and is then screened, larger agglomerated material going to the furnace for extraction of the metal and the fines and dust being recycled through the sintering machine.

Heretofore, the conventional processing of finely divided feed stock to effect agglomeration by sintering has employed the pug mill or similar apparatus to treat the feed stock containing the finely divided phosphatic materials, recycle fines, sinter dust, carbonaceous fuel, water, and any other desired component for the entire milling period. It is known that the moisture content of the feed material is the prior art process is relatively critical in that a deviation of about one percent in either direction causes a substantial decrease in furnaceable sinter production rate. The recycle fines are also very abrasive and it is therefore necessary to replace the cutter blades of the pug mill at frequent intervals when the aforesaid ma terials are simultaneously passed through the pug mill. Sinter feed material prepared in this manner lacks porosity thereby inducing a rather low burning rate and limits the depth of feed material to rather shallow beds.

One object of this invention is to provide an improved process of sintering metal-bearing materials whereby a substantial increase in sinter production is obtained.

Another object of this invention is to provide an improved method of sintering metal-bearing materials whereby a substantially broader range of feed moisture tolerance is obtained.

Still another object of this invention is to provide an improved method of sintering metal-bearing materials whereby the sinter bed has increased porosity which permits faster burning rates enabling the use of deeper sinter beds with resultant greater productive capacity of the sinter machine.

.A further object of this invention relates to an improved method of sintering wherein the feed material is characterized by having improved heat-transfer properties,

A still further object of this invention relates to an improved method of sintering wherein there is less abrasionof. the cutter blades of the pug mill per unit of furnaceable sinter.

Other objects and advantages of this invention Will be apparent to those skilled in the art from the following disclosure.

Now it has surprisingly been found that Withholding the recycle fines and carbonaceous fuel until. the balance of the sinter feed material has passed through or substantially passed through the pug mill, or other suitable milling apparatus, which functions in a similar manner tothe pug mill, i. e., having a cutting, chopping or breaking up action of the clayey matrix and also blending the matrix with the concentrates, phosphatic sand,

sinter recycle dust, or other metal-bearing materials employed, results in numerous unexpected advantages. Thus, it was found that the addition of the carbonaceous fuel and recycle fines separately or together when the pugging operation was completed or substantially completed (1) provided a substantially higher production rate of furnaceable sinter, (2) enabled a broader range of moisture content of the sinter feed material without deleteriouseffectto the production rate or sinter quality thereby-affording a broad operating tolerance, (3) resulted in better porosity of the sinter'bed, (4) permitted the burning of deeper sinter beds, (5) improved the heat-transfer characteristics of the sinter feed,-(6) reduced the abrasion of the cutter blades of the pug'mill perunit of furnacea'ble sinter, and (7) provided more latitude for controlof the sinter process variables.

'In accordance with this invention the desired quantity of recycle fines and carbonaceous fuel can be added separately or as .a substantially uniform "mixture to the balance of the sinter feed material in the pug mill by providing "an inlet at the discharge end of the pug mill just over the last cutter blades whereby the recycle fines and fuel are mixed into the sinter feed during the last few revolutions of the pug mill and are blended, but not substantially pugged, with the sinter feed. The recycle fines and fuel are not abraded and are not coated with mud, which would result in the formation of pellets wherein the core would be the recycle fines or fuel, as would be the case in the prior art processes. The recycle fines and carbonaceous fuel can also be blended with the balance of the sinter feed material after it is discharged from the pug mill by effecting brief mixing in a suitable mixer. Also a portion of the fuel can be withheld during the blending operation and can then be directly added to the top of the sinter feed material as a top dressing which will facilitate rapid and uniform ignition of the sinter feed material.

The sinter feed material prepared in accordance with this invention is charged to the sintering machine and sintered in the usual manner as hereinabove more fully described.

' The following examples 'are illustrative of the improved method of sintering metal-bearing materials.

Pilot plant equipment was initially employed to evaluate the instant invention.

The laboratory pug mill had a 24-inch diameter shell, was 32 inches long, and rotated at 9.1 R. P. M. Inside of this shell was an eccentrically mounted shaft containing four rows of 5-inch cutter blades, 12 blades per row, which rotated in the opposite direction from the shell at 177 R. P. M.

The pilot plant sinter pot consisted of an 18 x 18-inch pot, 11 inches deep, and had sides which tapered downward to a 14.5 x 14.5-inch grate for all experiments in Table I, but had vertical sides for all experiments in Table II. The grate was made from a pallet taken from the commercialsinter machine. Beneath the grate was a l2-inch wind box with a 4-inch exhaust main about which the pot was pivoted so that it could be inverted to dump the charge after it had been sintered. Air was pulled through the wind box by a blower fan, at a controlled rate by means of an orifice to measure the gas flow rate and a valve to control the gas flow through the wind box. Above the sinter pot was a retractable ignition hood which was fired by a propane-air mixture.

The sinter feed materials for the following experiments tabulated in Tables -I and II were prepared from stocks of sinter grade phosphate matrix, phosphatewasher concentrates, sinter dust, recycle fines, and anthracite fines which were stored in air-tight drums. Samples of these materials were weighed according to the feed proportions desired (percentages based on dry weights), the moisture content of the sinter feed was adjusted by the addition of the desired amount of water to the'mixture,in the pug mill just before the feed was pugged, and

then mixed for 66 seconds in the aforesaid batch-type, laboratory pug mill.

All of the experimental sinter feed materials were prepared in the'same manner except those experiments where the recycle fines or recyclefines and fuel were withheld from the pug mill and admixed with the feed after the pugging operation. In these latter cases the recycle fines and fuel were blended with the pugged feed by simultaneously pouring the pugged feed and dry stock through a 1-inch sample splitter. After the first split the two halves were simultaneously poured into a large pan, then again passed through the sample splitter and again combined.

The sinter feeds in each of Tables I and II were sintered in the same manner. The desired weight of sinter feed, prepared as aforesaid, was charged into the sinter pot and gently leveled. Then a small amount (109 grams) of coke fines was uniformly sprinkled over the top of the charge to assure its complete ignition and the ignition hood placed over the sinter pot. Full suction was then produced in the wind-box by opening the air-flow control valve, and the gas flow rate and windbox suction were recorded. The sinter feed was ignited for 40 seconds using a flame temperature of '1350 C., after which the ignition hood was removed. .The blower fan was turned off exactly five minutes after ignition was commenced. The sinter pot was then inverted and the hot sinter cake dumped into-'a heavy metal container. After the charge recovered was weighed, the sinter cake was scraped free of allnon-sintered material and reweighed. Then the cooled sinter cake was crushed in a 10-inch roll crusher'Ms-inch roll spacing. The crushed fragments were screened through %-inch and S-mesh screens, and the quantity of plus 8-meshsinter material used as a criterion of sinter quality.

. In all of the above experiments the total charge to the sinter pot was 72 pounds, th'e'amount of recycle fines was 20 percent, the matrix-concentrates ratio was 30-70, and the fuel was pugged.

TABLE Ir Recycle Fines Recycle Fines Added After and Fuel Added Pugging, Fuel After Pugging Pugged Percent water 18. 1 18. 6 17. 4 18. 6 18.2 17. 9 Percent carbon 5.0 6. 9 9. 1 4. 9 5. 3 9. 7

Avg. gas flow after ignition,

C F M 153 130 117 161 121 134 In all of the above experiments the total charge to the sinter pot was 80 pounds, the amount of recycle fines was 15' percent,and the matrix-concentrates ratio was It was subsequently determined on a plant scale that the addition of recycle fines after the pugging operation resulted in an increased production of about 17 percent over the prior practice of pugging'the entire sinter feed material. It was also found that the addition of recycle fines and fuel after'the' pugging operation resulted in an additional increased production of about 10 percent. Accordingly, it is readily apparent that this invention provides substantial economic advantages of increased production from existing equipment in addition to providing numerousv otheradvantages concerning desirable variations in processing conditions;

In the practice of this invention the carbonaceous fuel can be coke, coal, lignite, peat, charcoal and the like, coke being generally preferred. The fuel is relatively finely divided preferably being less than about 6-rnesh. The quantity of fuel will vary depending on the material selected and any fuel values contained in the ores being treated, but on the basis of fixed carbon will generally range from about 6 to about 12 percent and preferably from about 7 to about 8 percent by dry weight of the sinter feed material.

The recycle fines can be varied over a wide range of from about to about 25 percent or higher, but from about 10 to about 20 percent, and preferably about percent by dry weight of the sinter feed material is generally employed. Obviously the very low range will not provide an optimum sinter bed structure, but Will still provide a material improvement over prior art methods employing a like amount of recycle fines. On the other hand the high range of recycle fines is not normally employed as such practice materially reduces the furnaceable product and thereby reduces the net production of the sinter machine.

The water content of the sinter feed material can vary within the range of from about 5 to about 21 percent by wet weight, but is generally held within the limits of from about 16 to about 20 percent. It is still desirable to prepare the sinter feed material to contain about 18.5 percent by Weight of water, on a wet basis, but the production of furnaceable sinter is not critically affected by minor moisture content variations as in the prior art methods, thus the range of 16 to 20 percent water will now provide substantially the same production of furnaceable sinter.

The balance of the sinter feed material Will consist essentially of the finely divided metal-bearing material, e. g., phosphatic fines such as phosphate concentrate, phosphate matrix, sinter dust, etc.

I claim:

1. In a process of preparing finely divided metal-bearing ores for sintering the steps which comprise pug milling the finely divided metal-bearing ore with Water and subsequently briefly blending therewith recycle fines from a previous sintering operation and a carbonaceous fuel having a particle size of less than 6-mesh, such that the said fines and fuel components of the sinter feed material composition substantially retain their size and shape relationship and are substantially free from a coating of the pug-milled mixture, to provide a sinter feed material containing from about 5 to about 21 percent water, on a wet basis; from about 5 to about 25 percent recycle fines, on a dry basis; and from about 6 to about 12 percent of carbonaceous fuel, on a dry basis; all by weight of the sinter feed material.

2. The process of claim 1 wherein the metal-bearing ore is a phosphatic material.

3. The process of claim 2 wherein the sinter feed material contains from about 16 to about 20 percent water, on a wet basis, and from about 10 to about 20 percent recycle fines, on a dry basis.

4. The process of claim 3 wherein the carbonaceous fuel is coke.

5. The process of claim 4 wherein the sinter feed material contains from about 7 to about 8 percent by weight, on a dry basis, of coke.

6. A method of sintering phosphatic materials, which comprises intimately pug milling a mixture of finely divided phosphatic material and water, and subsequently briefly blending said mixture with (1) recycle fines from a previous sintering operation and (2) a carbonaceous fuel having a particle size of less than 6-mesh, such that the said fines and fuel components of the sinter feed material composition substantially retain their size and shape relationship and are substantially free from a coating of the pug-milled mixture, to provide a sinter feed material containing from about 16 to about 20 percent water, on a wet basis; from about 10 to about 20 percent recycle fines, on a dry basis; and from about 6 to about 12 percent of carbonaceous fuel, on a dry basis; all by Weight of the sinter feed material and characterized by improved porosity and feed moisture tolerance; thereafter feeding the sinter feed material to a traveling grate and contouring the surface thereof With a gate member to provide a substantially uniform moving sinter bed which is ignited at the surface thereof and caused to be burned downwardly through the length of the moving sinter bed at progressively increasing depths by means of a down-draft of air through the sinter bed thereby blast roasting the sinter feed material at a temperature sufiicient to effect agglomeration thereof, and crushing the completely smtered composition to provide furnace feed material of up to about 3-inch size.

7. The process of claim 6 wherein the carbonaceous fuel is coke.

8. The process of claim 7 wherein the sinter feed material contains from about 7 to about 8 percent by Weight, on a dry basis, of coke.

References Cited in the file of this patent UNITED STATES PATENTS 1,303,411 Swart et a1 May 13, 1919 2,248,180 Moriarty July 8, 1941 2,412,104 Stewart Dec. 3, 1946 

1. IN A PROCESS OF PREPARING FINELY DIVIDED METAL-BEARING ORES FOR SINTERING THE STEPS WHICH COMPRISES PUGMILLING THE FINELY DIVIDED METAL-BEARING ORE WITH WATER AND SUBSEQUENTLY BRIEFLY BLENDING THEREWITH RECYCLE FINES FROM A PREVIOUS SINTERING OPERATION AND A CARBONACEOUS FUEL HAVING A PARTICLE SIZE OF LESS THAN 6-MESH, SUCH THAT THE SAID FINES AND FUEL COMPONENTS OF THE SINTER FEED MATERIAL COMPOSITION SUBSTANTIALLY RETAIN THEIR SIZE, AND SHAPE RELATIONSHIP AND ARE SUBSTANTIALLY FREE FROM A COATING THE PUG-MILLED MIXTURE, TO PROVIDE A SINTER FEED MATERIAL CONTAINING FROM ABOUT 5 TOABOUT 21 PERCENT WATER, ON A WET BASIS; FROM ABOUT 5 TO ABOUT 25 PERCENT RECYCLE FINES, ON A DRY BASIS; AND FROM ABOUT 6 TO ABOUT 12 PERCENT OF CARBONACEOUS FUEL, ON A DRY BASIS; ALL BY WEIGHT OF THE SINTERS FEED MATERIAL. 